This invention relates to valve assemblies for internal combustion engines. More particularly, this invention relates to so-called "L-Head" valve assemblies for small internal combustion engines.
Increased environmental concerns and regulatory action have caused engine designers to look for new ways to reduce noxious emissions from gasoline-powered internal combustion engines. Specifically, engine designers are attempting to reduce the concentrations of carbon monoxide (CO) and nitrous oxide (NO.sub.x) gases that are exhausted from the combustion chamber of the engine.
One cause of noxious emissions is the improper seating, or the delay in proper seating, between the exhaust valve seating surface and the valve seat interconnected with the engine. Any improper seating or delay in proper seating of the exhaust valve will tend to increase the concentrations of unburned fuel and noxious gases that escape from the combustion chamber into the atmosphere. Also, the improper seating or delay in proper seating will tend to increase the amount of noxious crankcase gases which pass from the crankcase to the environment.
One cause of this improper seating is that the valve seat tends to tilt at engine running temperatures due to temperature gradients that exist across the exhaust valve seat. The temperature across the valve seat is greatest on the side of the valve which is closest to the combustion chamber, due to the higher temperatures that exist in the combustion chamber. As the distance increases from the combustion chamber, the temperature across the valve seat tends to decrease. The problem is more acute in L-Head engines due to the close proximity of the exhaust valve to the combustion chamber in such engines when compared to overhead valve (OHV) engines. In addition, the valves of L-Head engines are more difficult to cool than the valves in overhead valve engines due to space limitations.
Whenever the valve seat tilts, some misalignment occurs between the seating surface on the valve head and the valve seat. This misalignment results in improper seating, or a delay in proper seating, of the exhaust valve. Although the explosion forces during the next engine cycle may eventually force the exhaust valve to completely close, the delay in proper closing allows noxious gases to be exhausted into the atmosphere.